Internal combustion engines typically include an engine block having a cylinder head attached to the engine block. A head gasket is disposed between the engine block and the cylinder head. The cylinder head defines an oil return passage for returning oil from the cylinder head back to an oil sump, through a drain-back passage defined by the engine block. The engine block may further define a Positive Crankcase Ventilation (PCV) passage that is in fluid communication with both the oil return passage and the drain-back passage. The PCV passage communicates gasses from the oil return passage to a PCV separator chamber that is defined by the internal combustion engine. The oil return passage of the cylinder head is disposed vertically above the PCV passage and the drain-back passage, which allows oil draining from the oil return passage into the drain-back passage to inadvertently drain into the PCV passage, and thereby into the PCV separator chamber.